" Is this a hardware compatibility problem? "
Doubtful. More likely a connection has come loose or a card worked its way out.
Did you open it up and reseat all the parts?
Is the RAM missing? I see people salvage stuff like that.
I am trying to start a Compaq pentium II computer from 1998. I am using various hardware(monitor,keyboard, mouse)that didn't come with the computer. When I press the power button I get three long beeps each followed by a short beep. The third beep is followed by to short beeps. The screen is blank but I can hear the tube warm up when I plug the cable into the CPU. The keyboard Caplock lite is on. The screen is blank. The computer worked fine before it was stored but that was using Compaq monitor and keyboard. Is this a hardware compatibility problem? Thankyou for whatever advice you have.